1. Field of the Invention
The present invention relates to an injection apparatus for a motor-driven injection molding machine. In particular, it relates to an improvement that facilitates the control of back pressure during charging a process.
2. Description of the Related Art
FIG. 4 shows an injection apparatus of a conventional motor-driven injection molding machine. The reference numeral 1 denotes an injection apparatus. A barrel unit 3 fed with plastic material is mounted on an injection carriage 2. The barrel unit 3 includes a barrel having an internal elongated cylindrical bore. A screw 4 is rotatably and axially movably disposed in the bore of the barrel 3.
A ball screw 5 extends coaxially and integrally from a rear end portion of the screw 4. A spline shaft 6 is integrally connected to the rear end of the ball screw 5.
An injection motor 7 which is for use in the injection process and drives the ball screw 5 is installed in the injection carriage 2. A charging motor 8 which is for use in the charging process and drives the spline shaft 6 is as well installed in the injection carriage 2. A rotor 9 of the injection motor 7 is designed to act as a ball screw nut that engages with the ball screw 5. And a rotor 10 of the charging motor 8 is designed to act as a spline nut that is fitted onto the spline shaft 6. A numerical control unit 11 controls the operation of the injection motor 7 and charging motor 8 so that the screw 4 moves forwardly in the injection process, or rotates in the charging process.
During the charging process, the spline shaft 6 permits the transmission of the rotation of the charging motor 8 to the screw 4 integral with the spline shaft 6 and ball screw 5. The screw 4 rotates and plasticizes the material fed into the barrel 3. The melt material accumulates in front of the screw 4, forcing back the screw 4 rearwardly.
During the injection process, the combination of the ball screw 5 and ball nut 9 converts the rotation of the injection motor 7 to the liner motion which causes the screw 4 to move forwardly(the leftward direction in the figure), whereby the screw 4 injects the molten material accumulated in front of the screw 4 into a mold cavity. The numerical control unit 11 controls the injection motor 7 to rotate and controls the charging motor 8 to keep from rotating, so as to enable the screw 4 to move forwardly.
The movement of the screw 4 in the charging process consists of the rotation for plastication and the retreat motion caused by the pressurized molten material accumulated in front of the screw 4. The rotation of the screw 4 simultaneous with retreat is properly achieved, in conjunction with the injection motor 7 to control of the back pressure to be applied on the screw 4.
However, in this prior-art injection apparatus, the ball screw 5 is made integral with the spline shaft 6 and it is so arranged that the charging motor 8 rotates the spline nut 10 fitted onto the spline shaft 6. Therefore, while the charging motor 8 rotates the spline shaft 6 to cause the screw 4 to rotate, the ball screw 5 rotates inevitably. This inevitable rotation of the ball screw 5 brings the screw 4 to move rearwardly, regardless of the amount of the molten material forced forwardly by the rotating screw 4.
The prior-art injection apparatus provided with the charging motor 8 having such a structure that the spline shaft 6 passes through the rotor 10 involves following drawbacks to control the back pressure. The rotation of the rotor 10 prevents the spline shaft 6 from slipping axially through the rotor 10. That needs to apply a appropriate back pressure to the screw 4 to cause it to retreat at the velocity corresponding to the feed rate of the molten material. For that reason, it is difficult to control the charging motor 8 and injection motor 7 with the rotation of the former being synchronized with that of the latter. That fails in the precise control of the back pressure applied to the screw 4. As a result, The screw 4 is forced to retreat only by the pressure of the molten material.